Thermostable and corrosion-resistant cast nickel-chromium alloy

ABSTRACT

A nickel-chromium casting alloy comprising up to 0.8% of carbon, up to 1% of silicon, up to 0.2% of manganese, 15 to 40% of chromium, 0.5 to 13% of iron, 1.5 to 7% of aluminum, up to 2.5% of niobium, up to 1.5% of titanium, 0.01 to 0.4% of zirconium, up to 0.06% of nitrogen, up to 12% of cobalt, up to 5% of molybdenum, up to 6% of tungsten and from 0.01 to 0.1% of yttrium, remainder nickel, has a high resistance to carburization and oxidation even at temperatures of over 1130° C. in a carburizing and oxidizing atmosphere, as well as a high thermal stability, in particular creep rupture strength.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of prior filed copending PCTInternational application no. PCT/EP2004/000504, filed Jan. 22, 2004,which designated the United States and on which priority is claimedunder 35 U.S.C. §120, the disclosure of which is hereby incorporated byreference, and which claims the priority of German Patent Application,Serial No. 103 02 989.3, filed Jan. 25, 2003, pursuant to 35 U.S.C.119(a)-(d).

BACKGROUND OF THE INVENTION

The present invention relates to a thermostable and corrosion-resistantcast nickel-chromium alloy.

Nothing in the following discussion of the state of the art is to beconstrued as an admission of prior art.

High-temperature processes, for example those used in the petrochemicalindustry, require materials which are not only heat-resistant but alsosufficiently corrosion-resistant and in particular are able to withstandthe loads imposed by hot product and combustion gases. For example, thetube coils used in cracking and reformer furnaces are externally exposedto strongly oxidizing combustion gases with a temperature of up to 1100°C. and above, whereas a strongly carburizing atmosphere at temperaturesof up to 1100° C. prevails in the interior of cracking tubes, and aweakly carburizing, differently oxidizing atmosphere prevails in theinterior of reformer tubes at temperatures of up to 900° C. and a highpressure. Moreover, contact with the hot combustion gases leads tonitriding of the tube material and to the formation of a layer of scale,which is associated with an increase in the external diameter of thetube by a few percent and a reduction in the wall thickness by up to10%.

By contrast, the carburizing atmosphere inside the tube causes carbon todiffuse into the tube material, where, at temperatures of over 900° C.,it leads to the formation of carbides, such as M₂₃C₆, and, withincreasing carburization, to the formation of the carbon-rich carbideM₇C₃. The consequence of this is internal stresses resulting from theincrease in volume associated with the carbide formation ortransformation and a decrease in the strength and ductility of the tubematerial. Furthermore, graphite or dissociation carbon may form in theinterior of the tube material, which can, in combination with internalstresses, lead to the formation of cracks, which in turn cause morecarbon to diffuse into the tube material.

Consequently, high-temperature processes require materials with a highcreep strength or limiting rupture stress, microstructural stability andresistance to carburization and oxidation. This requirement is—withinlimits—satisfied by alloys which, in addition to iron, contain 20 to 35%of nickel, 20 to 25% of chromium and, to improve the resistance tocarburization, up to 1.5% of silicon, such as for example thenickel-chromium steel alloy 35Ni25Cr-1.5Si, which is suitable forcentrifugally cast tubes and is still resistant to oxidation andcarburization even at temperatures of 1100° C. The high nickel contentreduces the diffusion rate and the solubility of the carbon andtherefore increases the resistance to carburization.

On account of their chromium content, at relatively high temperaturesand under oxidizing conditions the alloys form a covering layer ofCr₂O₃, which acts as a barrier layer preventing the penetration ofoxygen and carbon into the tube material beneath it. However, attemperatures over 1050° C., the Cr₂O₃ becomes volatile, and consequentlythe protective action of the covering layer is rapidly lost.

Under cracking conditions, carbon deposits are inevitably also formed onthe tube inner wall and/or on the Cr₂O₃ covering layer, and attemperatures of over 1050° C. in the presence of carbon and steam, thechromium oxide is converted into chromium carbide. To reduce theassociated adverse effect on the resistance to carburization, the carbondeposits in the tube have to be burnt from time to time with the aid ofa steam/air mixture, and the operating temperatures generally have to bekept below 1050° C.

The resistance to carburization and oxidation is further put at risk bythe limited creep rupture strength and ductility of the conventionalnickel-chromium alloys, which lead to the formation of creep cracks inthe chromium oxide covering layer and to the penetration of carbon andoxygen into the tube material via the cracks. In particular in the eventof a cyclical temperature loading, covering layer cracks may form andalso the covering layer may become partially detached.

Tests have revealed that microstructural phase reactions, in particularat higher silicon contents, for example of over 2.5%, evidently lead toa loss of ductility and to a reduction in the short-time strength.

It would therefore be desirable and advantageous to inhibit the damagemechanism of carburization—production in the creep rupture strength orlimiting rupture stress—internal oxidation, with the further result ofincreased carburization and oxidation, and to provide an improvedcasting alloy which still has a reasonable service life even underextremely high operating temperatures in a carburizing and/or oxidizingatmosphere.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a nickel-chromiumcasting alloy having defined aluminum and yttrium contents andcomprising up to 0.8% of carbon up to 1% of silicon up to 0.2% ofmanganese 15 to 40% of chromium 0.5 to 13% of iron 1.5 to 7% of aluminumup to 2.5% of niobium up to 1.5% of titanium 0.01 to 0.4% of zirconiumup to 0.06% of nitrogen up to 12% of cobalt up to 5% of molybdenum up to6% of tungsten 0.01 to 0.1% of yttrium remainder nickel.

The total content of nickel, chromium and aluminum combined in the alloyshould be from 80 to 90%.

It is preferable for the alloy, individually or in combination with oneanother, to contain at most 0.7% of carbon, up to 30% of chromium, up to12% of iron, 2.2 to 6% of aluminum, 0.1 to 2.0% of niobium, 0.01 to 1.0%of titanium, up to 0.15% of zirconium and—to achieve a high creeprupture strength—up to 10% of cobalt, at least 3% of molybdenum and upto 5% of tungsten, for example 4 to 8% of cobalt, up to 4% of molybdenumand 2 to 4% of tungsten, if the high resistance to oxidation is not theprimary factor. Therefore, depending on the loads encountered in thespecific circumstances, the cobalt, molybdenum and tungsten contentshave to be selected within the content limits specified by theinvention.

An alloy comprising at most 0.7% of carbon, at most 0.2, more preferablyat most 0.1% of silicon, up to 0.2% of manganese, 18 to 30% of chromium,0.5 to 12% of iron, 2.2 to 5% of aluminum, 0.4 to 1.6% of niobium, 0.01to 0.6% of titanium, 0.01 to 0.15% of zirconium, at most 0.6% ofnitrogen, at most 10% of cobalt, and at most 5% of tungsten, isparticularly suitable.

Optimum results can be achieved if, in each case individually or incombination with one another, the chromium content is at most 26.5%, theiron content is at most 11%, the aluminum content is from 3 to 6%, thetitanium content is over 0.15%, the zirconium content is over 0.05%, thecobalt content is at least 0.2%, the tungsten content is over 0.05% andthe yttrium content is 0.019 to 0.089%.

The high creep rupture strength of the alloy according to the invention,for example a service life of 2000 hours under a load of from 4 to 6 MPaand a temperature of 1200° C., guarantees that a continuous, securelybonded oxidic barrier layer is retained in the form of an Al₂O₃ layerwhich has the effect of preventing carburization and oxidation, resultsfrom the high aluminum content of the alloy and continues to top itselfup or grow. As tests have shown, this layer comprises α-Al₂O₃ andcontains at most isolated spots of mixed oxides, which do not alter theessential nature of the α-Al₂O₃ layer; at higher temperatures, inparticular over 1050° C., in view of the rapidly decreasing stability ofthe Cr₂O₃ layer of conventional materials at these temperatures, isincreasingly responsible for protecting the alloy according to theinvention from carburization and oxidation. On the Al₂O₃ barrier layer,there may also—at least in part—be a covering layer of nickel oxide(NiO) and mixed oxides (Ni(Cr,Al)₂O₄), the condition and extent ofwhich, however, is not of great significance, since the Al₂O₃ barrierlayer below is responsible for protecting the alloy from oxidation andcarburization. Cracks in the covering layer and the (partial) flaking ofthe covering layer which occurs at higher temperatures are thereforeharmless.

To ensure that the α-aluminum oxide layer is as pure as possible andsubstantially free of mixed oxides, the following condition should besatisfied:9[% Al]≧[% Cr].

On account of its high aluminum content, the microstructure of the alloyaccording to the invention, at over 4% of aluminum, inevitably containsγ′ phase, which has a strengthening action at low and mediumtemperatures but also reduces the ductility or elongation at break. Inindividual cases, therefore, it may be necessary to reach a compromisebetween ductility and resistance to oxidation/carburization which isoriented according to the intended use.

The barrier layer according to the invention comprising α-Al₂O₃, whichis the most stable Al₂O₃ modification, is able to withstand all oxygenconcentrations.

BRIEF DESCRIPTION OF THE DRAWING

Other features and advantages of the present invention will be morereadily apparent upon reading the following description of currentlypreferred exemplified embodiments of the invention with reference to theaccompanying drawing, in which:

FIG. 1 shows a graphical illustration of various alloys, illustratingthe elongation limit as a function of the temperature;

FIG. 2 shows a graphical illustration of the alloys, illustrating thetensile strength as a function of the temperature;

FIG. 3 shows a graphical illustration of the alloys, illustrating theelongation at break as a function of the temperature;

FIG. 4 shows a graphical illustration of alloys, illustrating the loadas a function of the Larson-Miller parameter/100;

FIG. 5 shows a graphical illustration of other alloys, illustrating theload as a function of the Larson-Miller parameter/100;

FIG. 6 shows a graphical illustration of still other alloys,illustrating the load as a function of the Larson-Miller parameter/100;

FIG. 7 shows a graphical illustration of comparative tests betweenalloys according to the invention and standard alloys at a temperatureof 1100° C.;

FIG. 8 shows a graphical illustration of alloys, illustrating theincrease in weight as a function of time;

FIGS. 9 and 10 show graphical illustrations of alloys, illustrating theincrease in weight as a function of cycles;

FIG. 11 shows a graphical illustration of test results of alloys withregard to influence of preliminary oxidation on the carburizationbehavior;

FIG. 12 shows a graphical illustration of alloys, illustrating theincrease in weight as a function of time between an alloy according tothe invention and standard alloys;

FIG. 13 shows a graphical illustration of contents of the alloyaccording to the invention,

FIG. 14 show a graphical illustration of a comparison between steelalloys according to the invention and alloys; and

FIGS. 15 and 16 show graphical illustrations of an alloy according tothe invention with respect to influence of the aluminum.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Throughout all the Figures, same or corresponding elements are generallyindicated by same reference numerals. These depicted embodiments are tobe understood as illustrative of the invention and not as limiting inany way. It should also be understood that the drawings are notnecessarily to scale and that the embodiments are sometimes illustratedby graphic symbols, phantom lines, diagrammatic representations andfragmentary views. In certain instances, details which are not necessaryfor an understanding of the present invention or which render otherdetails difficult to perceive may have been omitted.

The invention is explained in more detail below on the basis ofexemplary embodiments and the seven comparative alloys 1 to 7 and ninealloys 8 to 26 according to the invention listed in the table below, andalso the diagrams shown in FIGS. 1 to 16. Alloy C Si Mn P S Ni Cr Mo Fe 1 0.44 1.72 1.23 0.014 0.005 34.4 25.02 0.01 35.91   2 0.38 0.57 0.540.009 0.001 32.2 19.9  <0.01   remainder 0.52 2.20 1.64 0.025 0.013 36  26.52 0.33  3 0.53 2.05 0.29 0.014 0.004 30.4 29.94 0.02 35.32   4 0.462.03 1.26 0.018 0.004 45.7 34.35 0.01 14.85   5 0.03 n.d. n.d. n.d. n.d.78.5 n.d n.d. 3.0   6 0.09 2.13 1.14 0.017 0.004 35.1 26.02 0.01 33.25  7 0.20 0.25 0.05 n.d. n.d. remainder 25.00 n.d. 9.50  8 0.42 0.09 0.060.004 0.001 remainder 25.70 0.01 9.70  9 0.42 0.10 0.06 0.005 0.001remainder 25.35 0.01 9.95 10 0.42 0.01 0.16 0.010 0.001 remainder 25.850.07 9.02 11 0.44 0.05 0.19 0.010 0.002 remainder 30.40 0.07 10.71  120.45 0.03 0.16 0.010 0.001 remainder 25.60 0.07 9.23 13 0.45 0.06 0.160.010 0.001 remainder 26.70 0.08 9.25 14 0.40 0.04 0.16 0.010 0.001remainder 25.10 0.08 9.15 15 0.41 0.08 0.14 0.010 0.010 remainder 25.850.08 9.01 16 0.41 0.06 0.13 0.011 0.001 remainder 25.40 0.08 9.15 170.48 0.06 0.13 0.010 0.001 remainder 25.80 0.08 8.95 18 0.44 0.05 0.130.010 0.001 remainder 25.65 0.08 8.95 19 0.42 0.05 0.13 0.010 0.001remainder 25.80 0.07 8.90 20 0.43 0.06 0.13 0.010 0.001 remainder 25.400.09 8.75 21 0.51 0.08 0.13 0.010 0.001 remainder 26.15 0.07 9.05 220.64 0.07 0.14 0.009 0.001 remainder 25.70 0.07 8.45 23 0.44 0.06 0.040.004 0.001 remainder 26.40 0.07 0.95 24 0.42 0.05 0.03 0.004 0.001remainder 26.10 3.92 0.39 25 0.47 0.06 0.04 0.005 0.001 remainder 22.300.11 4.30 26 0.39 0.01 0.05 0.005 0.001 remainder 26.05 3.56 7.20 AlloyV W Cu Co Nd Ti Zr Y Al B N  1 0.03 0.04 0.03 0.01 0.84 0.10 0.02 n.d.0.13 0.0003 0.039  2 0.03 <0.01   0.01 n.d. 0.51 <0.01   <0.01  <0.01    <0.01 n.d. 0.016 0.12 0.82 0.09 1.28 0.26 0.20 0.03 0.115  30.04 0.04 0.03 0.01 1.02 0.06 0.05 n.d. 0.07 0.0004 0.072  4 0.04 0.010.02 0.05 0.96 0.10 0.03 n.d. 0.00 0.0018 0.107  5 n.d. n.d. n.d. n.d.n.d. n.d. n.d. n.d. 4.5 n.d. n.d.  6 0.03 0.04 0.03 0.01 0.98 0.02 0.01n.d. 0.01 0.0054 0.084  7 nd. n.d. 0.05 n.d. n.d. 0.15 0.05 0.085 2.1n.d. n.d.  8 0.01 0.13 0.01 0.06 1.06 0.15 0.08 0.019 2.3 n.d. n.d.  90.01 0.12 0.02 0.06 0.99 0.13 0.06 0.055 2.5 n.d. 0.055 10 0.02 0.060.05 0.10 0.03 0.13 0.05 0.028 2.5 0.0033 0.052 11 0.02 0.05 0.05 0.090.10 0.14 0.05 0.024 2.4 0.0034 0.060 12 0.02 0.06 0.05 0.09 0.53 0.120.05 0.029 2.3 0.0033 0.049 13 0.02 0.06 0.05 0.09 1.00 0.14 0.05 0.0282.4 0.0041 0.050 14 0.02 0.06 0.06 0.10 0.03 0.15 0.05 0.025 3.6 0.00380.039 15 0.04 0.06 0.03 0.05 1.10 0.19 0.07 0.070 3.8 0.0023 0.034 160.04 0.07 0.03 0.03 2.07 0.17 0.06 0.066 3.7 0.0008 0.043 17 0.04 0.070.03 0.04 1.15 0.18 0.08 0.061 3.9 0.0005 0.042 18 0.04 0.82 0.03 0.051.09 0.18 0.06 0.066 3.7 0.0005 0.038 19 0.04 0.06 0.03 0.04 1.11 0.180.05 0.061 3.3 0.0004 0.047 20 0.04 0.06 0.02 0.05 1.05 0.16 0.06 0.0554.8 0.0020 0.034 21 0.04 0.08 0.03 0.05 1.10 0.16 0.07 0.047 3.0 0.00040.047 22 0.04 0.06 0.02 0.04 1.00 0.18 0.06 0.046 3.1 0.0004 0.033 230.02 0.03 0.01 0.04 1.06 0.16 0.08 0.049 3.4 0.0004 0.052 24 0.03 0.040.01 8.35 1.00 0.16 0.01 0.045 3.7 0.0011 0.048 25 0.02 4.50 0.01 8.201.00 0.22 0.05 0.047 3.6 0.0010 0.031 26 0.03 1.26 0.01 0.61 0.09 0.170.01 0.044 2.6 0.0012 0.058

The table includes, as an example for two wrought alloys which are notcovered by the invention and have a comparatively low carbon content anda very fine-grained microstructure with a grain size of ≦10 μm,comparative alloys 5 and 7, whereas all the other test alloys arecasting alloys.

Yttrium has a strong oxide-forming action which, in the alloy accordingto the invention, considerably improves the formation conditions andbonding of the α-Al₂O₃ layer.

The aluminum content of the alloy according to the invention has animportant role in that aluminum leads to the formation of a γ′precipitation phase, which significantly increases the tensile strength.As can been seen from the diagrams presented in FIGS. 1 and 2, the yieldstrength and the tensile strength of the three alloys according to theinvention 13, 19, 20 to 900° C. are well above the correspondingstrengths of the four comparative alloys. The elongation at break of thealloys according to the invention substantially correspond to that ofthe comparative alloys; it increases considerably above approximately900° C., as can be seen from the diagram presented in FIG. 3, while thestrength reaches the level of the comparative alloys (FIGS. 1, 2). Thiscan be explained by the fact that above approximately 900° C. the γ′phase starts to form a solution, and is completely dissolved at aboveapproximately 1000° C.

The limiting rupture strength of alloys according to the invention withdifferent aluminum contents is presented in the Larson-Miller diagramshown in FIG. 4. Absolute temperatures (T in ° K) and service life untilfracture (t_(B) in h) are linked to one another by the Larson-Millerparameter LMP:LMP=T·(C+log₁₀(t _(B))).

According to the illustration presented in FIG. 4, different aluminumcontents lead to different service lives until fracture. The limitingrupture stress of the alloys according to the invention are muchsuperior to those of conventional oxidation-resistant wrought alloys(FIG. 5). If alloys according to the invention are compared withconventional centrifugally cast materials, similar service lives untilfracture are observed in the temperature range of around 1100° C.

In the range around 1200° C., i.e. with greater Larson-Millerparameters, there are no known service life data for conventionalcentrifugally cast materials, whereas limiting rupture stresses of from5.8 to 8.5 MPa are still observed for the alloys according to theinvention for service lives of 1000 h, depending on the composition.

Further tests, in which the resistance to carburization of variousspecimens was tested in a slightly oxidizing atmosphere comprisinghydrogen and 5% by volume of CH₄, reveal the superiority of the alloyaccording to the invention compared to four standard alloys at atemperature of 1100° C. The long-time performance is of particularimportance. The test results are presented in graph form in the diagramshown in FIG. 7. It can be seen from this diagram that the two alloysaccording to the invention 8 and 14 have carburization resistance whichremains constant over the course of time, and that in the case of alloy14 comprising 3.55% of aluminum, this is even better than in the case ofalloy 8 with an aluminum content of just 2.30%. The diagram presented inFIG. 8 shows the carburization over the course of time as the increasein weight for the alloy according to the invention 11 containing 2.40%of aluminum compared to the four standard alloys 1, 3, 4 and 6, withmuch lower aluminum contents. This figure likewise reveals thesuperiority of the alloy according to the invention.

To simulate practical conditions, cyclical carburization tests werecarried out, in which the specimens were alternatively held at atemperature of 1100° C. for 45 min and then at room temperature for 15min in an atmosphere comprising hydrogen together with 4.7% by volume ofCH₄ and 6% by volume of steam. The results of the tests, which eachcomprise 500 cycles, are shown in the diagram presented in FIG. 9.Whereas specimens 8, 14 in accordance with the invention experienced noor only a slight change in weight, the formation of scale and flaking ofthe scale led to considerable weight losses in the case of comparativespecimens 1, 3, 4, 6, and in the case of comparative specimen 1 afterjust approximately 300 cycles. Furthermore, the alloy 14 according tothe invention, with its higher aluminum content, once again revealsbetter corrosion properties than alloy 8, which is likewise covered bythe invention.

The results of further tests, in which the specimens were subjected tocyclical thermal loading at 1150° C. in dry air, are presented in thediagram shown in FIG. 10. The curves reveal the superiority of the testalloys according to the invention (top set of curves) compared to theconventional alloys (bottom set of curves), which were subject to aconsiderable weight loss after just a few cycles. The results indicate astable, securely bonded oxide layer in the case of the alloys accordingto the invention. To establish the influence of preliminary oxidation onthe carburization behavior, ten specimens of the alloy according to theinvention were exposed to an atmosphere comprising argon with a lowoxygen content at 1240° C. for 24 hours and were then carburized for 16hours at a temperature of 1100° C. in an atmosphere comprising hydrogencontaining 5% by volume of CH₄. The test results are presented in graphform in the diagram shown in FIG. 11, which also indicates thecorresponding aluminum contents. Accordingly, a slightly oxidizingannealing treatment reduces the resistance to carburization of thespecimens according to the invention up to an aluminum content of 3.25%(specimen 14); as the aluminum content rises further, the resistance tocarburization of the alloy which has been annealed in accordance withthe invention improves (specimens 16 to 19), while at the same time thediagram clearly reveals the poor carburization behavior of thecomparative specimens 1 (0.128% of aluminum) and 4 (0.003% of aluminum).The deterioration in the resistance to carburization at lower aluminumcontents can be explained by the fact that the inheritantly protectiveoxide layer cracks open or (partially) flakes off during cooling afterthe annealing treatment, so that carburization occurs in the region ofthe cracks and flaked-off areas. At higher aluminum contents, theabovementioned Al₂O₃ barrier layer is formed beneath the oxide layer(covering layer).

In a test carried out under conditions close to those encountered inpractice, a number of specimens were subjected to cyclical carburizationand decarburization in accordance with the NACE standard. Each cyclecomprised carburization for three hundred hours in an atmospherecomprising hydrogen and 2% by volume of CH₄, followed by decarburizationfor twenty-four hours in an atmosphere comprising air and 20% by volumeof steam at 770° C. The test comprised four cycles. It can be seen fromthe diagram presented in FIG. 12 that the specimen in accordance withthe invention 14 underwent scarcely any change in weight, whereas in thecase of comparative specimens 1, 3, 4, 6 a considerable increase inweight or carburization occurred, and this did not disappear even duringthe decarburization.

The diagram presented in FIG. 13 reveals that the contents in the alloyaccording to the invention should be matched to one another in such away that the following condition is satisfied:9[% Al]≧[% Cr].

The straight line in the diagram shown in FIG. 13 divides the range ofalloys with a sufficiently protective α-aluminum oxide layer above thestraight line from the range of alloys with a resistance tocarburization or catalytic coking which is adversely affected by mixedoxides.

The diagram illustrated in FIG. 14 reveals the superiority of the steelalloy according to the invention using six exemplary embodiments 21 to26 by comparison with the conventional comparative alloys 1, 3, 4, 6 and7. The compositions of the comparative alloys 21 to 26 are given in thetable.

To illustrate the influence of the aluminum within the content limitsaccording to the invention, the diagrams presented in FIGS. 15 and 16compare the service life of the alloy according to the invention 13,comprising 2.4% of aluminum, as reference variable, with service life 1,in each case at 1100° C. (FIG. 15) and 1200° C. (FIG. 16) for threeloading situations (15.9 MPa; 13.5 MPa; 10.5 MPa) with the service livesof the alloys according to the invention 19 (3.3% of aluminum) and 20(4.8% of aluminum) referenced on the basis of the above referencevariable.

The diagram shown in FIG. 15 reveals that in the case of alloy 19, witha medium aluminum content of 3.3%, the decrease in the service lifebecomes more intensive with increasing load, whereas in the case ofalloy 20, with its high aluminum content of 4.8%, there is a strong butapproximately equal decrease in the relative service life for all theloading situations. The diagram for 1200° C. reveals a reduction in theservice life when the aluminum content is increased from 2.4% (alloy 13)to 3.3% (alloy 19) for all three loading situations, with the relativeservice life dropping by approximately one third. A further increase inthe aluminum content to 4.8% (alloy 20) in turn reveals a load-dependentreduction in the relative service life.

Overall, the two diagrams reveal that as the aluminum content increases,the service life until fracture in the limiting rupture stress test isreduced. Furthermore, as the temperature increases and the duration ofloading increases and/or the loading level decreases, the negativeinfluence of the aluminum on the limiting rupture stress life decreases.In other words: the alloys with a high aluminum content are particularlysuitable for long-term use at temperatures for which it has hithertobeen impossible to use cast or centrifugally cast materials.

In view of their superior strength properties and their excellentresistance to carburization and oxidation, the casting alloy accordingto the invention is particularly suitable for use as a material forfurnace parts, radiant tubes for heating furnaces, rollers for annealingfurnaces, parts of continuous-casting and strip-casting installations,hoods and muffles for annealing furnaces, parts of large diesel engines,containers for catalysts and for cracking and reformer tubes.

While the invention has been illustrated and described in connectionwith currently preferred embodiments shown and described in detail, itis not intended to be limited to the details shown since variousmodifications and structural changes may be made without departing inany way from the spirit of the present invention. The embodiments werechosen and described in order to best explain the principles of theinvention and practical application to thereby enable a person skilledin the art to best utilize the invention and various embodiments withvarious modifications as are suited to the particular use contemplated.

1. A nickel-chromium casting alloy, comprising up to 0.8% of carbon upto 1% of silicon up to 0.2% of manganese 15 to 40% of chromium 0.5 to13% of iron 1.5 to 7% of aluminum up to 2.5% of niobium up to 1.5% oftitanium 0.01 to 0.4% of zirconium up to 0.06% of nitrogen up to 12% ofcobalt up to 5% of molybdenum up to 6% of tungsten 0.019 to 0.089% ofyttrium remainder nickel.


2. The nickel-chromium casting alloy as claimed in claim 1, comprisingat most 0.7% of carbon, at most 1% of silicon, up to 0.2% of manganese,18 to 30% of chromium, 0.5 to 12% of iron, 2.2 to 5% of aluminum, 0.4 to1.6% of niobium, 0.01 to 0.6% of titanium, 0.01 to 0.15% of zirconium,at most 0.06% of nitrogen, at most 10% of cobalt, at least 3% ofmolybdenum and at most 5% of tungsten, individually or in combinationwith one another.
 3. The nickel-chromium casting alloy as claimed inclaim 1, comprising at most 0.7% of carbon, at most 1% of silicon, up to0.2% of manganese, 18 to 30% of chromium, 0.5 to 12% of iron, 2.2 to 5%of aluminum, 0.4 to 1.6% of niobium, 0.01 to 0.6% of titanium, 0.01 to0.15% of zirconium, at most 0.06% of nitrogen, at most 10% of cobalt, upto 4% of molybdenum and at most 5% of tungsten, remainder nickel.
 4. Thenickel-chromium casting alloy as claimed in claim 1, comprising at most26.5% of chromium, at most 7% of iron, 3 to 6% of aluminum, over 0.15%of titanium, over 0.05% of zirconium, at least 0.2% of cobalt, up to 4%of molybdenum and over 0.05% of tungsten, individually or in combinationwith one another.
 5. The nickel-chromium casting alloy as claimed claim1, wherein the aluminum and chromium contents satisfy the followingcondition:9[% Al]≧[% Cr].
 6. The nickel-chromium alloy as claimed in claim 1,wherein a total content of nickel, chromium and aluminum ranges from 80to 90%.
 7. (canceled)
 8. A method of using a nickel-chromium castingalloy as claimed in claim 1 as a material for the manufacture of afurnace part, radiant tube for a heating furnace, roller for anannealing furnace, part of continuous-casting and strip-castinginstallations, hood and muffle for an annealing furnace, part of a largediesel engine, shaped body for a catalyst filling, and for cracking andreformer tubes.